Effect of visual experience on tubulin synthesis during a critical period of visual cortex development in the hooded rat.

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Abstract
In some species, restriction of visual experience in early life may affect normal functional development of visual cortical cells. Visual deprivation was used during post-natal development in the hooded rat to determine the affect on the production of certain brain cell molecular components, e.g., tubulin needed for the growth and maintenance of synapses and neurites. Norwegian black hooded rats were reared under a variety of conditions of visual deprivation. At various stages of development the animals were killed and the rate of synthesis of tubulin in visual and motor cortex determined. Tritiated colchicine was used to assay tubulin and L-[14C]leucine injected into the brain ventricles 2 h before death was used to measure rate of tubulin synthesis. In rats reared in normal light, a marked elevation in visual cortex tubulin synthesis spanned the period from eye-opening (13 days) until approximately 35 days. This elevation in tubulin synthesis was absent in animals reared in darkness from birth or deprived of pattern vision by eyelid suture. The effect of visual deprivation on tubulin synthesis was confined to visual cortex and not found for the motor cortex. The incorporation of L-[14C]leucine into total protein in visual cortex was unaffected by dark rearing. Stimulation of tubulin synthesis by visual experience in rat visual cortex was not from a general non-specific stimulation of protein synthesis. Rats dark-reared from birth and exposed to a lighted environment for 24 h during a critical period extending from eye-opening (13 days) until approximately 35 days, displayed a significant increase in visual cortex tubulin synthesis compared to control animals kept continuously in darkness. Rats brought into the light later than 35 days showed no significant increase in tubulin synthesis when compared with dark-reared controls. The number of synapses and cytoplasmic processes a developing cell can maintain apparently depends on the size of the tubulin pool available to that cell. Tubulin in brain only has a half-life of about 4 days, so when the level of tubulin drops, competition may result between different synapses for the limited supply of tubulin needed for their maintenance, contributing to the structural plasticity of the visual cortex during the critical period.